TVA shut down the Raccoon Mountain facility near Chattanooga in March after inspections found significant cracks in the rotors.
The generators pull water from the Tennessee River and pump it to fill a reservoir atop the mountain.
Then the water flows back down through those generators to make power during peak demand times. Hydropower is extremely cheap to produce and the plant was being used nearly every day.
The Chattanooga Times Free Press reported the federal utility expects repairs to cost $55 million. Completion could take more than a year.